The use of optical fiber communication systems has increased significantly during the last few years. It appears likely that the use of this mode of communications will continue to increase in the future. Companies engaged in the manufacture of components for these systems continue to seek ways to reduce the cost thereof and to handle efficiently materials invovled in the production thereof.
Presently, optical fibers are being manufactured in processes which include vapor deposition as a materials forming technique. This technique includes the reaction of a silicon-containing gas or vapor and the reaction of germanium-containing gas or vapor, for example, to form a deposited glass having suitable optical properties. These processes are used to manufacture preforms which is the first step in making lightguide fibers. One such process which is known as a modified chemical vapor deposition (hereinafter MCVD) process is described in J. B. MacChesney "Materials and Processes for Preform Fabrications -Modified Chemical Deposition", Vol. 64, proceedings of IEEE, pages 1181-1184 (1980).
Input to the MCVD process may comprise a carrier gas and reactant vapors such as germanium tetrachloride (GECI.sub.4), silicon tetrachloride (SiCI.sub.4), and phosphorous oxychloride (POCI.sub.3). These reactant vapors are supplied from vaporizers commonly referred to as deposition bubblers and are passed to a deposition site such as a glass substrate tube. A preform from which optical fiber is drawn is manufactured by heating the substrate tube to a temperature in the range of 1600.degree. to 1800.degree. C. to react the vapors and deposit them in a predetermined manner within the substrate tube. In the manufacture of preforms, the reactant vapors must be blended precisely and delivered at controlled concentration levels to the substrate tube. This can be done by bubbling a carrier gas such as oxygen, for example, through heated supplies of the reactant materials in liquid form in bubblers and then to the deposition site with the vapors entrained in the carrier gases.
Typically, a deposition bubbler includes a container in which a carrier gas intake conduit terminates in an orifice located below the free surface of liquid contained therein. An outlet conduit provides fluid communication between the space above the surface of the liquid and the vapor deposition site. Exemplary of deposition systems employing bubblers is that illustrated in U.S. Pat. No. 3,826,560.
Inasmuch as vapor of the liquid contained within a deposition bubbler is withdrawn during deposition, the level of liquid drops unless the bubbler is replensished from an auxiliary source. In some applications, decreases in the level of liquid within the bubbler have little effect. In other applications, however, such as in vapor deposition processes employed in the manufacture of optical fiber preforms, variations in the liquid level may have an adverse effect such as changing the concentration level of the delivered vapor. This is attributable to the fact that the rate of vaporization is not solely dependent upon the surface area of liquid within the bubbler, which area can be maintained constant by the use of cylindrically shaped containers. The vaporization rate also is dependent upon several other factors including the flow characteristics of the carrier gas bubbled through the liquid. For example, the size of the bubbles, as they rise through the liquid, has an effect on the rate of vaporization. The rate of flow of the carrier gas introduced into the bubbler also affects the rate of vaporization, as does the residence time of the bubbles which, of course, depends on the depth at which the carrier gas is introduced. Another factor is the control of the heat transfer into the bubbler which is affected by significant changes in the quantity of liquid in the bubbler. Although it is possible to program a heater controller to account for some of these variables as changes in the level of liquid are continuously monitored, that approach is complex and does not satisfy completely the need for vapor delivery control in the bubbler.
The prior art includes U.S. Pat. No. 4,235,829 which issued on Nov. 25, 1980 to Fred P. Partus. In it, there is shown a vapor delivery system which comprises a deposition bubbler adapted to generate and to deliver vapor from a liquid contained therein and in a reservoir in fluid communication with the bubbler. Facilities are provided for sensing the level of the liquid contained within the bubbler and for providing gaseous head pressures in the reservoir of magnitudes dependent upon the sensed liquid level. The liquid level in the bubbler drops as liquid is vaporized and withdrawn from the bubbler whereupon the level is adjusted by increasing the pressure head in the reservoir to feed liquid to the bubbler. Unfortunately, perturbations in the deposition bubbler caused by a drop in liquid level and then a rise due to the changing of the pressure in the reservoir can affect adversely the rate of vaporization and hence the concentration level of the vapor. These level changes are exacerbated as the rate of deposition and hence the rate of withdrawal of vapor are increased. The prior art also includes closed systems in which, for example, one bubbler is positioned within another with both bubblers being depleted substantially sequentially.
Seemingly, the prior art does not include a satisfactory solution for the problem of controlling the liquid level in a deposition bubbler in order to control the delivery of vapor to a deposition site such as a substrate tube used in the manufacture of optical preforms. In order to provide quality preforms, the delivery arrangement must include provisions for preventing perturbations in the deposition bubbler. This requirement becomes important, particularly in view of the higher deposition rates toward which the industry appears to be moving.